Optical fiber connectors are a critical part of essentially all optical fiber communication systems. For instance, such connectors are used to join segments of fiber into longer lengths, to connect fiber to active devices, such as radiation sources, detectors and repeaters, and to connect fiber to passive devices, such as switches, multiplexers, and attenuators. The principal function of an optical fiber connector is to hold the fiber end such that the fiber's core is axially aligned with an optical pathway of the mating structure. This way, light from the fiber is optically coupled to the optical pathway.
Of particular interest herein are “expanded beam” optical connectors. Such connectors are used traditionally in high vibration and/or dirty environments, where “physical contact” between the fiber and the light path of mating connector is problematic. Specifically, in dirty environments, particulates may become trapped between connectors during mating. Such debris has a profoundly detrimental effect on the optical transmission since the particles are relatively large compared to the optical path (e.g., 10 microns diameter in single mode) and are therefore likely to block at least a portion of the optical transmission. Furthermore, in high-vibration environments, optical connectors having ferrules in physical contact tend to experience scratching at their interface. This scratching diminishes the finish of the fiber end face, thereby increasing reflective loss and scattering.
To avoid problems of debris and vibration, a connector has been developed which expands the optical beam and transmits it over an air gap between the connectors. By expanding the beam, its relative size increases with respect to the debris, making it less susceptible to interference. Further, transmitting the beam over an air gap eliminates component-to-component wear, thereby increasing the connector's endurance to vibration. Over the years, the expanded beam connector has evolved into a ruggedized multi-fiber connector comprising an outer housing which is configured to mate with the outer housing of a mating connector, typically through a screw connection. Contained within the outer housing are a number of inner assemblies or “inserts.” Each insert comprises an insert housing, a ferrule assembly contained within the insert housing and adapted to receive a fiber, and a ball lens at a mating end of the insert housing optically connected to the fiber. The ball lens serves to expand and collimate light at the connector interface. When two expanded beam connectors are mated, there is an air gap between the ball lenses of each pair of optically coupled inserts.
Tyco Electronics Corporation (Harrisburg, Pa.) currently offers a line of expanded beam connectors under the brand name PRO BEAM® Referring to FIGS. 5(a) & (b), the single mode and multimode PRO BEAM® connectors 41, 42 are shown schematically. The single mode (SM) expanded beam connector 41 uses a PC-polished ferrule 43 that is in contact with a glass ball lens 44. (Note: a Physical Contact (PC) polish is slightly rounded, and the surface of the fiber is nominally perpendicular to the fiber axis. See, for example, Telcordia GR-326.) The lens 44 is AR coated on one side for a glass/glass interface, and, on the other side, for an air/glass interface.
The multimode (MM) connector 42 of FIG. 5(b) uses a flat-polished ferrule 45 which is held at a fixed distance from the ball lens 46 by means of a stop 47 that is located near the ball lens. The ball lens has an antireflective (AR) coating 48 for an air/glass interface to reduce Fresnel losses.
Therefore, the SM connector differs from the MM connector in that the ball lens and the ferrule are in physical contact, and the ball lens has a different AR coating on two sides.
Although the multimode and single mode expanded beam connectors offered by Tyco have consistently met industry requirements, applicants have identified a need for (1) improved performance, (2) lower costs, and (3) enhanced durability.
Practical limitations significantly limit the return loss of the current SM expanded beam connector. Specifically, the connector has a specified return loss of greater than 34 dB. To achieve this return loss, the ball lens must be coated with an AR coating that reduces reflections at the fiber/lens interface. Although reflection at this interface can be reduced theoretically to zero, typically only a reflectivity of 0.025% (36 dB return loss) is achieved. When the fiber is in contact with the ball lens, the index of refraction of the ball lens should be almost exactly 2 to achieve the lowest possible loss for a mated connector pair. The ball lens used, however, has an index that is slightly less than 2 at the SM wavelengths 1310 and 1550 nm. As a result, the loss is 0.3 dB higher than the ideal case.
Applicants also recognize that, to reduce inventory costs and requirements, the SM and MM connectors should share as many components as possible. Although the two current connectors are similar, the MM and SM inserts are distinct parts. Additionally, the inserts for different wavelength applications (e.g., 850 and 1300 nm) are also different parts since the distance between the ferrule and lens must be adjusted to accommodate different ball lens focal lengths which are a function of wavelength. The number of parts is further compounded by the fact that most varieties of the PRO BEAM™ connector are stocked in 1, 2, and 4 channel versions as well as in different form factors. The large variety of parts needed to fulfill the different connector permutations creates logistical problems that complicate supply chains and necessitate large inventories.
Applicants also recognize that using specialty components adversely affects the connector's cost and production time. Specifically, the SM connector uses a lens material that is more expensive than the lens used for the MM connector. This lens material is also more difficult to obtain since it is a specialty order.
For these reasons, a need exists for a family of MM and SM expanded beam connectors which are similar in design to reduce parts inventory, but also deliver the desired performance and vibration resistance. The present invention fulfills this need among others.